We have recently used (P32)-8-N3cAMP to successfully photo-label the cAMP binding sites in three different cell systems. These are the human erythrocyte, slime mole (Dictyostelium Discoideum) and Sarcoma 37. After photo-labeling the cells are solubilized and electrophoresized on SDS polyacrylamide gels. The gels are stained for protein, dried on filter paper and autoradiographed. The autoradiograph allows determination of: (1) which proteins bind cAMP (2) the MW of these proteins and (3) the effects of the other compounds and stage of life cycle on the level and type of cAMP binding proteins that are present. In addition, the effect of cAMP and N3 cAMP on the phosphorylation of these membrane proteins by (gamma-P32)-ATP are being studied in a like manner. BIBLIOGRAPHIC REFERENCES: Haley, B., "Photo-Affinity Labeling of cAMP Binding Sites on Human Red Cell Membranes", Biochemistry 14, 3852 (1975). Pomerantz, A., S. Rudolph, B. Haley and P. Greengard (1975), "Photo-Affinity Labeling of a Protein Kinase from Bovine Brain with 8-Azido-Adenosine 3 feet - 5 feet Monophosphate", Biochemistry 14, 3858.